1. Field of the Invention
The present invention relates to an optical pickup for high density recording/reproduction, and a method to detect a reproduction signal, and more particularly, to an optical pickup for high density recording/reproduction, and a method to detect a reproduction signal, which are capable of preventing degradation of a reproduction signal caused by shot noise, which occurs when a low optical power light is used to form a small light spot for high density recording/reproduction.
2. Description of the Related Art
In general, for high density optical recording/reproduction, a light spot focused on a recording medium must be small. The diameter of the light spot, which has a Gaussian distribution, is larger than a track pitch, so a portion of the light spot focused on a main track extends into neighboring tracks.
Thus, if an intensity of the light spot is strong, an information signal can be recorded on and/or erased from the neighboring tracks, and a reproduction signal of the main track is adversely affected by the neighboring tracks. To avoid these problems, as a recording density increases, a light source with a lower optical power is needed.
FIG. 1 illustrates an example of a conventional optical pickup 100. As shown in FIG. 1, the conventional optical pickup 100 includes a light source 1, an objective lens 5 for focusing light emitted from the light source 1 to form a light spot on an information recording surface of an optical disc 10, a beam splitter 3 disposed on an optical path between the light source 1 and the objective lens 5, to change the traveling path of incident light, and a photodetector 7 to receive the light beam passed through the objective lens 5 after having been reflected by the optical disc 10, and directed toward the photodetector 7 by the beam splitter 3, to detect an information signal and/or an error signal.
The conventional optical pickup 100 forms a single light spot on a main track of the optical disc 10 to record the information signal on or to reproduce the information signal from the information recording surface of the optical disc 10. However, the conventional optical pickup 100 having the above configuration for high density recording and reproduction needs a low optical power light in reproducing information. The use of the low optical power light during reproduction causes shot noise, thereby sharply degrading the quality of a reproduction signal.
The reason that the quality of the reproduction signal degrades due to shot noise caused by the low optical power light is rooted in the quantal properties of light. In other words, it is probable that a photon is absorbed and photoelectrically converted by the photodetector 7, so that a signal output from the photodetector 7 always contains shot noise ripples. If the optical power of the light source 1 is high, a large number of photons are received by the photodetector 7, so that a high power signal is output from the photodetector 7. In this case, a shot noise level of the photodetector 7 is small relative to an amplitude of the output signal, and thus the problem of shot noise is not significant. In contrast, if the optical power of the light source 1 is low, the number of photons absorbed by the photodetector 7 decreases. As a result, a low power signal is detected by the photodetector 7 and the problem of shot noise is significant. For example, to increase recording density by four times, the size of the light spot must be reduced by one fourth, and the shot noise level increases by about three times. In other words, when a low optical power light is used for reproduction, the quality of the reproduction signal degrades due to a low signal-to-noise ratio.